1. Field of the Invention
The present invention relates generally to tools for inserting and removing sensors such as pitot tubes into and from fluid conveying pipelines. More particularly, the present invention relates to a tool or apparatus for inserting and removing a sensor through a hot tap in a high pressure pipeline without interrupting the flow of fluid in the pipeline.
2. Description of the Prior Art
In fluid conveying pipelines, such as gas, water, or petrochemical pipelines, it is often desirable to collect information about the flow rate, pressure, or other parameters of fluid flowing in the pipeline by insertion of a measuring device such as a pitot tube, hereinafter sometimes referred to generally as a sensor, into the flowing fluid. For example, in order to measure flow, an elongated flow-sensor, such as the pitot tube described in U.S. Pat. No. 5,036,711, may be inserted into the flow. It is also desirable to insert and remove sensors from the pipeline while the fluid is present and flowing therein. This facilitates moving sensors from location to location, when it is desired to measure the characteristics of the fluid only periodically, and facilitates removal of the sensor for maintenance, cleaning and subsequent re-insertion.
In order to accomplish insertion and removal without interrupting flow, an arrangement known as a hot tap is widely used. A hot tap generally comprises a coupling through a wall of the fluid pipeline and an access valve connected to the outlet. The access valve is of a type which provides an unobstructed access passageway to the interior of the pipeline when the valve is open. The valve is connected by an access tube to a packing gland designed to seal around the sensor while permitting the sensor to be inserted and removed through the gland, tube, and valve.
Many problems are inherent in inserting and removing a sensor through a hot tap in a fluid pipeline. The pressure of the fluid within the pipeline may be extremely high, creating a substantial force resisting insertion of the sensor. In addition, the sensor is typically long and slender, making it difficult to handled and creating the possibility of binding at the packing gland and damaging the sensor if it is not inserted or removed substantially along a longitudinal axis of the access tube. Accordingly it is necessary to substantially align the sensor with the longitudinal axis of the access tube before inserting the sensor into the packing gland. This requires an unobstructed insertion space between the packing gland and the nearest obstruction, such as another pipe, a wall, or a ceiling, the insertion space being at least as long as the sensor and valve height. Such insertion and removal tools increase the required insertion spaced making it difficult to use in restricted spaces.
Several insertion and removal tools have been used to partially overcome these problems. Piston-driven hydraulic tools have been used to apply an insertion force through a drive rod to the top of the sensor, pushing the sensor into the pipeline. These tools can provide sufficient force but are mechanically complex, and thus prone to failure. In addition, since they must be aligned with the longitudinal axis of the sensor, these tools substantially add to the required insertion space.
Direct insertion tools using threaded stem and nut constructions have also been used. These tools are secured to the top of the sensor and provide a mechanical advantage for manual insertion, thus helping to overcome the high pressure of the fluid pipeline and being mechanically simpler than the hydraulic insertion devices. However, these tools share the disadvantage of requiring additional insertion space. Single and dual drive rod systems which pull the top of the sensor toward the access valve and the fluid pipeline have also been used An example of a single drive rod system is disclosed in U.S. patent application Ser. No. 07/850,760, assigned to the assignee of the present invention. However, such a single drive rod system may not provide sufficient force in a high-pressure pipeline situation. Thus, dual drive rod systems have been developed with a first plate operatively connected to the packing gland or access tube and a second plate operatively connected to the sensor. A pair of externally-threaded drive rods link the two plates. As each of the drive rods is rotated, the two plates are pulled together. An illustrative dual drive rod system is manufactured by Dieterich Standard, Product No. DHF-25-GH6SA.
There are, however, many drawbacks to such a dual drive system. First, insertion and removal with this system is slow because each of the two drive rods are rotated separately. As the first and then the second drive rod are alternately rotated, bending moments are caused through the tool. Bending moments may affect the sensor by damaging it or causing it to hang up in the access tube, valve or at the pipe wall. In addition, by placing the sensor in a position that is not perpendicular to the flow of fluid, inaccurate measurements may be obtained. Second, the bending moments may be translated to the drive rods causing galling of their threads. Third, the bending moments cause greater friction at the packing gland thereby increasing the force required to insert the sensor. In addition, with this alternating operation of the drive rods, each drive rod is alternately placed in a position of bearing the entire load and twisting due to the applied torque, thus defeating the purpose of a dual drive rod system.
Worm gear dual drive rod systems which rotate a threaded hub around a fixed threaded rod have been developed to eliminate some of these problems. Such systems include a crank and worm gear which mates with drive gears and gear boxes associated with each of the drive shafts in a conventional fashion. However, these systems require critical alignment between the drive shaft, worm gears, drive gears and gear boxes. In addition, the "timing" of drive gears is extremely difficult since a worm gear apparatus offers little allowance for gear backlash and misalignment. These misalignment problems are further enhanced by the fact that threaded drive rods of appreciable length have relatively large tolerances on concentricity, straightness and bow.
A dual drive rod apparatus is disclosed in U.S. Pat. No. 5,009,113. The pair of threaded drive rods operatively interconnect a measuring means and a valve so as to allow the measuring means to alternatively be driven toward or away from the valve. A combination of a crank, a pair of cogged wheels, and a cogged belt is used to rotate a pair of threaded bushings attached to the measuring means and each engaging with one of the stationary drive rods. This apparatus suffers from the disadvantage of having no free play to accommodate drive rods which are not straight.
An additional problem associated with all single and dual drive rod apparatus is the inability of the user to know when the sensor is in the proper position in the pipe. This can result in inaccurate measurements if the sensor is not inserted far enough into the pipeline. Furthermore, systems such as the worm gear type described above have an enormous mechanical advantage which makes it possible to unknowingly drive the sensor into the opposite wall of the fluid pipeline, thus causing damage to the sensor or rupture of the pipeline.
It is against this background and prior art that the present invention has been developed to provide an improved apparatus for inserting and removing sensors.